Typically, cardiac stimulation leads comprise a hollow insulating sleeve in a flexible material with an internal electric conductor (two conductors in the case of a bipolar lead), finished at its distal extremity by a bearing surface which is intended to come in contact with the endocardium. The bearing surface is equipped with an electrode (the so-called "distal" electrode in the case of a bipolar lead), making it possible to carry out the stimulation of the myocardium. The distal stimulation electrode generally presents a flattened frontal extremity, constituting an active surface, in touching contact with the wall of the myocardium. The design of such an extremity of the lead must satisfy various requirements, which, until now, have been contradictory.
The first requirement, which is essential, is to provide a high impedance at the heart/electrode interface, in order to decrease the current necessary for the stimulation and, consequently, to increase the lifespan of the pulse generator. To increase the interface impedance, it is desirable to reduce the dimensions of the active surface of the stimulation electrode (see, in particular, Clementy et al. Economies d'energie: le role de la sonde [Energy Saving: The Role of the Lead], Stimucoeur 1998, 26 no. 4, pp. 184-187).
However, a reduction in the dimensions of the lead extremity involves an increase in the pressure at the heart/electrode interface leading to increases in the stimulation threshold, and potentially to perforations of the myocardium.
In one particular known geometry, the stimulation electrode is a ring which has a flattened annular surface a flattened frontal (distal) end, whose central area is insulated. One can thus have a current conducting surface without reducing the total surface area (conducting and non-conducting) bearing against the endocardium. This structure reflects a first compromise solution as between the aforementioned constraints.
In addition, the choice of a material for the electrode, such as a microporous vitreous carbon in the place of a metal, e.g., platinum, makes it possible to combine an excellent biocompatibility with satisfactory electric performance (in particular, low energy losses by polarization). However, even in this case, the contact impedance remains relatively low, about 500 Ohm.
Further, the annular shape of the electrode leads to losses of current directly in blood, through the part of the ring which is not in contact with the myocardium. This constitutes an additional factor, prejudicial to the lifespan of the pulse generator, from the reduction in the impedance of contact.